1. Field of the Invention
The present invention relates to a quartz crystal unit that is configured with a quartz crystal blank incorporated inside a receptacle, and more particularly to a crystal unit for high-frequency use in which the thickness of the crystal blank in the vibration region is reduced from the thickness in the outer periphery.
2. Description of the Background Art
Crystal units that are configured with crystal blanks housed inside receptacles are well known as frequency control elements, and are incorporated in the oscillation circuits of various electronic apparatuses as reference sources of frequency and time. In a crystal unit that employs an AT-cut quartz crystal blank, which is a representative quartz crystal blank, the resonance frequency is inversely proportional to the thickness of the crystal blank.
As optical communications have come into widespread use in recent years, great quantities of crystal units are being incorporated in optical communication systems, and this application has encouraged a trend toward crystal units with higher oscillation frequencies. To meet the need for higher oscillation frequencies, crystal units are being developed in which a depression is provided in the vibration region of the crystal blank to reduce the thickness of the crystal blank in this depression, whereby the oscillation frequency is raised, and moreover, the vibration region is held and the mechanical strength maintained by the relatively thick portions around the periphery of the depression. As an example, Japanese Patent Laid-Open Publication No. 2004-72676 (JP, P2004-72676A) discloses a crystal unit in which a portion of the crystal blank is removed by etching to reduce the thickness, the area in which thickness has been reduced serving as the vibration region.
FIG. 1A is a plan view of a crystal blank that is used in a crystal unit of the prior art, FIG. 1B is a sectional view of this crystal blank, and FIG. 1C is a sectional view of the crystal unit that incorporates this type of crystal blank.
The crystal unit is provided with, for example, rectangular crystal blank 1, which is an AT-cut quartz crystal blank. Crystal blank 1 has a thin central portion that is circular vibration region 1a and a thicker peripheral portion that encloses the outer circumference of vibration region 1a. In other words, a cylindrical depression is formed in one major surface of crystal blank 1, and the bottom of this depression is vibration region 1a. A pair of mutually opposed excitation electrodes 2a and 2b is formed on the both major surfaces of the crystal blank in this vibration region 1a. Excitation electrodes 2a and 2b are formed separated from positions that contact the inner walls of the depression for forming vibration region 1a, and are formed such that the overall area of vibration region 1a is sufficiently large compared to the areas of the excitation electrodes. Extending electrodes 3a and 3b are formed to extend from the pair of excitation electrodes 2a and 2b toward two end portions on opposite sides of the crystal blank. Extending electrodes 3a and 3b are formed on both major surfaces of crystal blank 1, respectively. Extending electrode 3a, which is formed on the major surface in which the depression of the crystal blank is formed, is formed folded back over the other major surface of the opposite side at the end portion of the crystal blank.
This crystal blank is fabricated by lithographic processes and etching processes, and a plurality of crystal blanks 1 is fabricated from a single crystal wafer (not shown in the figure). As shown in FIG. 1C, crystal unit 1 is held in receptacle 4 by securing the both ends of crystal blank at which extending electrodes 3a and 3b are extended on the inside bottom surface of surface-mounting receptacle 4 by conductive adhesive 7. A pair of internal terminals for electrically connecting to extending electrodes 3a and 3b are provided on the inside bottom surface of receptacle 4, and these internal terminals extend to mounting terminals 6 on the outer surface of receptacle 4. Cover 5 closes the opening of receptacle 4 that accommodates crystal blank 1, whereby crystal blank 1 is hermetically sealed inside receptacle 4. This completes the configuration of a crystal unit for surface mounting.
If the oscillation frequency in the fundamental wave of a crystal unit is to be 622 MHz, the thickness of the crystal blank in vibration region 1a is made approximately 2.2 μm. In addition, in response to demand for miniaturization of the overall crystal unit, the outer planar dimensions of crystal blank 1 are, for example, 1.4 mm×1.2 mm; and vibration region 1a is formed as a circle having a diameter of 0.5 mm. Each of excitation electrodes 2a and 2b is formed as, for example, a circle having a diameter of 0.2 mm that is positioned at the center of vibration region 1a. 
However, in a crystal unit having the above-described configuration, the problem arises that the crystal Impedance (CI) basically increases due to the decrease in the outside dimensions of the crystal blank that is imposed by the miniaturization of the crystal unit. For example, decrease in the size of excitation electrodes 2a and 2b causes a reduction of the length of the electrically connected regions, i.e., the connection length, between excitation electrodes 2a and 2b and extending electrodes 3a and 3b. In this case, the connection length is the length of the portions in which excitation electrodes and extending electrodes are in contact in the direction that is orthogonal to the direction in which excitation electrodes and extending electrodes are joined, i.e., the direction of flow of a high-frequency current. When the connection length is small, the electrical conductive resistance between the excitation electrodes and the extending electrodes is great, and sufficient electrical energy therefore cannot be supplied from extending electrodes 3a and 3b to excitation electrodes 2a and 2b. In addition, the widths of extending electrodes 3a and 3b also become smaller, and this factor also contributes to an increase in the conductive resistance. Such factors that cause an increase in the conductive resistance in turn cause the crystal impedance of the crystal unit to increase. Typically, when the crystal impedance in a crystal unit is great, the oscillation characteristics deteriorate.
However, in the above-described crystal unit that is disclosed in JP, P2004-72676A, circular excitation electrodes 2a and 2b are used, and the corresponding extending electrodes are each led out with approximately half of the periphery on mutually opposite sides of circular excitation electrodes 2a and 2b as the connection regions, as shown in FIGS. 2A and 2B, and the connection length between the excitation electrodes and extending electrodes is thus increased and the conductive resistance decreased. Extending electrodes 3a and 3b are formed so as to spread out fan-like on mutually opposite sides with the excitation electrodes as center. The width and thickness of extending electrodes 3a and 3b are also increased to further reduce the conductive resistance. By adopting this configuration, the crystal impedance in the crystal unit shown in FIGS. 2A and 2B can be reduced to a low level.
According to another conceivable configuration for further reducing the crystal impedance, each of extending electrodes 3a and 3b is connected by the entire periphery of excitation electrodes 2a and 2b to increase the connection length between extending electrodes 3a and 3b and excitation electrodes 2a and 2b. In this way, electrical energy is supplied to excitation electrodes 2a and 2b from the entire periphery of the excitation electrodes. However, when extending electrodes are connected to excitation electrodes around the entire periphery in this way, the pair of extending electrodes 3a and 3b overlap each other with the crystal blank interposed, giving rise to problems that are described below.
FIG. 3 shows an equivalent circuit diagram of a crystal unit. The equivalent circuit of a typical crystal unit is represented as a configuration in which equivalent parallel capacitance C0 is parallel-connected to the serial connection of: equivalent series inductance L, equivalent series capacitance C1, and equivalent series resistance R. In addition, the ratio of equivalent parallel capacitance C0 to equivalent series capacitance C1 is referred to as “capacitance ratio γ.” In an oscillation circuit that uses a crystal unit, the variable width of frequencies typically increases as capacitance ratio γ decreases, and the configuration and adjustment of the oscillation circuit is therefore facilitated. The crystal unit is therefore normally designed such that capacitance ratio γ is small.
When the pair of extending electrodes 3a and 3b overlap each other with the crystal blank interposed, excitation electrodes 2a and 2b become substantially larger, and the equivalent parallel capacitance C0 that is chiefly caused by inter-electrode capacitance therefore increases, causing capacitance ratio γ to increase. Equivalent parallel capacitance (inter-electrode capacitance) C0 and equivalent series capacitance C1 both increase in proportion to the opposition area of excitation electrodes 2a and 2b, but the increase of equivalent series capacitance C1 is limited. When the connection of extending electrodes 3a and 3b to the entire outer circumferences of excitation electrodes 2a and 2b essentially increases the sizes of excitation electrodes 2a and 2b, equivalent series capacitance C1 is saturated and equivalent parallel capacitance C0 still increases, ultimately causing capacitance ratio γ to increase.
If capacitance ratio γ of an appropriate range is to be obtained, the connection length between each excitation electrode and the corresponding extending electrode must be limited to up to half of the outer circumference of the excitation electrode, and this imposes limits on the increase of the connection length and the reduction of the crystal impedance.